HAssOs on Mac Mini

Hello everyone.
As you can see from the (many) questions I am a novice user.
I installed Home Assistant OS on a MAC-Mini (early 2009) deleting the previous iOS.
When I start the PC I no longer know how to enter the bios if I want to set the USB boot priority again to install a new OS.

The boot process shows me a screen with Slots A and B.
Always used the first line not knowing what the others do, do you have any clarifications for me?

I can’t find the grub.cfg file which I understand is responsible for the boot. I can’t find it because it isn’t there, in fact the find command doesn’t find anything. What do you recommend?

The installation did not find the drivers for the wifi, not that it is essential but I would like to learn how to solve a problem like this.

Thanks^^